In the field of optical disks for recording sounds, images, and the like, a plurality of formats such as CD, DVD, and BD have been developed in recent years for the purpose of increasing recording capacity. This requires optical disk driving apparatuses for driving optical disks to be accommodated to optical disks of the plural formats. In view of the circumstance, there have been developed optical pickup apparatuses employing a package of two-wavelength laser device that emits laser light of two wavelength types (see publicly-known document 1 (“Japanese Unexamined Patent Publication No. 2004-22051 (published on Jan. 22, 2004)”)).
FIG. 7 shows a configuration of an optical system in an optical pickup apparatus 101 employing a two-wavelength laser device.
The optical pickup apparatus 101 includes, as shown in the figure, a two-wavelength laser diode 103, a collimator lens 104, a beam splitter 105, an objective lens 107, a spot lens 108, and a light receiving amplification element 110.
Laser light emitted from the two-wavelength laser diode 103, which is a light emitting device serving as a light source for recording and reproduction, is changed into parallel light by the collimator lens 104. Thereafter, an optical path of the laser light thus changed is bent by 90° by the beam splitter 105, and then illuminates an optical disk 102 via the objective lens 107. Thereafter, light reflected from the optical disk 102 is converged by the spot lens 108 via the objective lens 107 and the beam splitter 105, and then comes incident on the light receiving amplification element 110. On the basis of incident light signals, the light receiving amplification element 110 reproduces information signals and generates focusing servo signals and/or focusing servo signals. The light receiving amplification element 110 then supplies a signal processing circuit (not illustrated), a control circuit (not illustrated), and the like with the signals thus reproduced and generated.
Meanwhile, the two-wavelength laser diode 103 emits laser light of two wavelength-types, depending upon the type of the optical disk 102 (in the present case, CD laser light having a wavelength of 780 nm and DVD laser light having a wavelength of 650 nm are emitted). In the two-wavelength laser diode 103, emission points from which the laser lights are emitted, respectively, are approximately 100 μm apart. In the optical pickup apparatus 101, a common optical system is used regardless of laser light. Therefore, in a same manner as the emission points in the two-wavelength laser diode 103, the laser lights come incident on locations that are approximately 100 μm apart, respectively, on the light receiving amplification element 110. Accordingly, in the case where the two-wavelength laser diode is employed, the light receiving amplification element needs to include a light receiving section for each laser light. In the present case, two light receiving sections, one for CD and the other one for DVD, need to be included.
FIG. 8 shows a light receiving section of the light receiving amplification element 110. As described above, light receiving sections (light receiving section 110A for DVD, and light receiving section 110B for CD) for the laser lights are provided, respectively. If, for example, a three-wavelength laser diode is developed in the future, obviously light receiving sections need to be included. Note that, as shown in the figure, each of the light receiving sections includes a main light-receiving section for information signals and a sub light-receiving section for tracking servo signals.
FIG. 9 shows a configuration of a light-receiving amplifier circuit 121 included in the light receiving amplification element 110. The light-receiving amplifier circuit 121, as shown in the figure, is configured with a previous amplifier circuit 122 and a following amplifier circuit 123.
The previous amplifier circuit 122, in the present case, includes two main light-receiving sections a (light receiving section for DVD) and A (light receiving section for CD), and an output changing switch S1 that causes an output of one of the main light-receiving sections a and A to be supplied to the following amplifier circuit 123.
The main light-receiving section a includes a photodiode PD1 and a trans impedance type amplifier circuit that is configured with an amplifier A1, which converts current generated in the photodiode PD1 into voltage and outputs the voltage, a feedback resistor R1, and a feedback capacitor C1. An anode of the photodiode PD1 is grounded. A cathode of the photodiode PD1 is connected to an input terminal of the amplifier A1. Further, the feedback resistor R1 and the feedback capacitor C1 are connected parallel to the amplifier A1. An output terminal of the amplifier circuit A1 is connected to the output changing switch S1.
The main light-receiving section A has a same configuration as the main light-receiving section a. The main light-receiving section A includes a photodiode PD2 and a trans impedance type amplifier circuit that is configured with an amplifier A2, which converts current generated in the photodiode PD2 into voltage and outputs the voltage, a feedback resistor R2, and a feedback capacitor C2. An anode of the photodiode PD2 is grounded. A cathode of the photodiode PD2 is connected to an input terminal of the amplifier A2. Further, the feedback resistor R2 and the feedback capacitor C2 are connected in parallel to the amplifier circuit A2. An output terminal of the amplifier circuit A2 is connected to the output changing switch S1.
The following amplifier circuit 123 includes an amplifier A3 and resistors R3 to R5.
One input terminal of the amplifier A3 is connected to one end of the resistor R3. The other end of the resistor R3 is connected to the output changing switch S1. One end of the resistor R4 is connected to a connection point of the one input terminal of the amplifier A3 and the one end of the resistor R3. The other end of the resistor R4 is connected to a terminal via which a reference voltage is externally supplied. The other input terminal of the amplifier A3 is connected to an output terminal of the amplifier A3 via the resistor R5. The output terminal of the amplifier A3 is an output terminal of the light-receiving amplifier circuit 121.
In the case where DVD laser light is emitted from the two-wavelength laser diode 103, reflected light from the optical disk 102 is supplied to the main light-receiving section a, and then converted from current into voltage. At this time, the output changing switch S1 is switched in a manner such that an output of the main light-receiving section a is supplied to the following amplifier circuit 123. Therefore, the output of the main light-receiving section a is supplied to the following amplifier circuit 123, amplified at the following amplifier circuit 123, and then outputted.
On the other hand, in the case where CD laser light is emitted from the two-wavelength laser diode 103, reflected light from the optical disk 102 is supplied to the main light-receiving section A, and then converted from current into voltage. An output of the main light-receiving section A is supplied to the following amplifier circuit 123 via the output changing switch S1, which is switched in a manner such that an output of the main light-receiving section A is supplied to the following amplifier circuit 123. Thereafter, the output of the main light-receiving section A is amplified at the following amplifier circuit 123 and then outputted.
Note that, concretely, the amplifier A1 of the main light-receiving section a is configured with a differential amplifier circuit that amplifies a difference between an output of the photodiode PD1 and the reference voltage supplied externally, and then outputs the difference thus amplified. In a same manner, the amplifier A2 of the main light-receiving section A is configured with a differential amplifier circuit that amplifies a difference between an output of the photodiode PD2 and the reference voltage supplied externally, and then outputs the difference thus amplified.
Meanwhile, to allow the light-receiving amplifier circuit 121, which is capable of accommodating the two-wavelength laser diode, to be included in an optical pickup apparatus for high-speed reproduction/recording, improvement in response frequency characteristics, output offset voltage, and output noise characteristics is required. To improve these respective characteristics, it is effective that each of the amplifiers A1 and A2, which is conventionally configured with a differential amplifier circuit, is configured with a grounded emitter amplifier circuit. The following describes reasons therefor.
First, the response frequency characteristics will be described, with reference to FIG. 10.
FIG. 10 shows response frequency characteristics of the respective amplifiers A1 and A2. Note that the response frequency characteristics of the respective amplifiers A1 and A2 are decided in accordance with formula (1) below:fc=1/(2πRC) (Hz)  (1)where R is a resistance of the feedback resistor R1 (or feedback resistor R2), and C is a capacitance of the feedback capacitor C1 (or feedback capacitor C2). Further, the feedback circuit in the figure corresponds to the feedback resistor R1 and the feedback capacitor C1 in the amplifier A1, and the feedback resistor R2 and the feedback capacitor C2 in the amplifier A2.
As shown in the figure, the amplifiers A1 and A2 have different response frequency characteristics from each other. Thus, limits of response frequencies as a result of feedback become frequencies f1 and f2, respectively.
At this time, even if the feedback capacitor C1 is set small in order to improve the response frequency characteristics of the amplifier A1, it is not possible to obtain a response equal to or greater than a certain frequency because the amplifier A1 is configured with a differential amplifier circuit (because the response frequency characteristics are slow in the case of open-loop). This is the same in the amplifier A2. Therefore, to improve the response frequency characteristics of the light-receiving amplifier circuit 121, it is required to set the feedback resistor R1 (or feedback resistor R2) small, or to improve the response frequency characteristics of the amplifiers in the case of open-loop.
Next, the following describes the output offset voltage. The output offset voltage is voltage that is outputted when no input signal is supplied to the light receiving section (when no reflected is supplied light from the optical disk 102).
It is ideal that the output offset voltage matches with the reference voltage supplied externally to the following amplifier circuit 123. However, an offset voltage is generated in the previous amplifier circuit 122 due to manufactural deviation and the like. The output offset voltage of the light-receiving amplifier circuit 121 is voltage generated at the following amplifier circuit 123 by amplifying the offset voltage of the previous amplifier circuit 122. Therefore, to restrain deviation in the output offset voltage, it is effective to reduce a gain of the following amplifier circuit 123.
Next, the following describes the output noise characteristics. In a same manner as the output offset voltage, it is effective with respect to the output noise characteristics to reduce a gain of the following amplifier circuit 123, because noise generated at the previous amplifier circuit 122 is amplified at the following amplifier circuit 123.
As described above, to improve the respective characteristics of the light-receiving amplifier circuit 121, it is required to improve the response frequency characteristics of the amplifiers, included in the previous amplifier circuit 122, in the case of open-loop, and to set a gain of the previous amplifier circuit 122 greater, as much as possible, than the gain of the following amplifier circuit 123.
Meanwhile, the grounded emitter amplifier circuit has excellent response frequency characteristics in the case of open-loop, and a characteristic of a wide dynamic range of outputs. Therefore, to improve the respective characteristics, it is effective that each of the amplifiers of the previous amplifier circuit 122 is configured with the grounded emitter amplifier circuit.
FIG. 11 shows an exemplary circuit in which the previous amplifier circuit is actually configured with a grounded emitter amplifier circuit.
A previous amplifier circuit 5 is configured with the photodiode PD1 (light receiving device), an grounded emitter amplifier circuit 1, an active load and bias circuit 2 of the grounded emitter amplifier circuit 1, an output circuit 3 configured with an emitter follower circuit, a feedback resistor R31, and a feedback capacitor C31.
The grounded emitter amplifier circuit 1 includes an NPN-type transistor (grounded emitter transistor) Q1. The active load and bias circuit 2 of the grounded emitter amplifier circuit 1 include PNP-type transistors Q3 and Q5, resistors R1 and R2, and a constant-current source I2. The output circuit 3 includes an NPN-type transistor Q4 and a constant-current source I1.
In the active load and bias circuit 2 of the grounded emitter amplifier circuit 1, a base of the transistor Q3 and a base of the transistor Q5 are connected. The base of the transistor Q5 is connected to a collector of the transistor Q5. The collector of the transistor Q5 is grounded via the constant-current source I2.
An emitter of the transistor Q3 is connected to one end of the resistor R2. An emitter of the transistor Q5 is connected to one end of the resistor R1. The other end of the resistor R1 and the other end of the resistor R2 are connected to a power supply Vcc.
An anode of the photodiode PD1 is grounded. A cathode of the photodiode PD1 is connected to a base of the transistor Q1. A collector of the transistor Q1 is connected to a collector of the transistor Q3. A connection point of the collector of the transistor Q1 and the collector of the transistor Q3 is connected to a base of the transistor Q4.
A collector of the transistor Q4 is connected to the power supply Vcc. An emitter of the transistor Q4 is grounded via the constant-current source I1. The feedback resistor R31 and the feedback capacitor C31, which is connected in parallel to the feedback resistor R31, are connected across the base of the transistor Q1 and a connection point of the emitter of the transistor Q4 and the constant-current source I1. The base of the transistor Q1 is an input terminal of the previous amplifier circuit 5. The connection point of the emitter of the transistor Q4 and the constant-current source I1 is an output terminal Vo of the previous amplifier circuit 5.
Meanwhile, to allow the previous amplifier circuit 5 configured with the grounded emitter amplifier circuit as described above to be included as the previous amplifier circuit 122, which is capable of accommodating a two-wavelength laser diode, of the light-receiving amplifier circuit 121 of the conventional technique described above, it is necessary to include two light receiving sections. In order to do so, it is necessary to include a plurality of the circuit surrounded by the dashed-dotted line in FIG. 11. This causes a big problem of increase in the number of elements. Note that a conventional technique is found in publicly-known Document 2 (“Japanese Unexamined Patent Publication No. 2001-202646 (published on Jul. 27, 2001)”).